AGE AND SEX COMPOSITION OF THE STRIPED DOLPHIN DIE-OFF IN THE WESTERNMEDITERRANEAN

During the morbillivirus epizootic that affected the Mediterranean str iped dolphins (Stenella coeruleoalba) in 1990-1992, several thousand i ndividuals are believed to have died. In order to determine the segmen ts of the population that were affected by the event, the sex- and age -frequency distributions of the animals that were washed ashore on the Spanish coasts were studied. The age of the animals was determined by counting dentine growth layer groups in the teeth. The results obtain ed indicate that both sexes were affected similarly by the disease and that the population component that suffered the largest mortality was that of sexually mature individuals, although substantial mortality o f calves, probably still dependent on their mothers, was also observed . juveniles, in contrast, appeared to be only slightly affected by the epizootic. This pattern of mortality is not consistent with die epide miology of morbillivirus infections previously observed in other mamma ls. Possible explanations of this particular age distribution include behavioral factors, increased susceptibility to the disease in adults due to the effects of pollution, high temperatures and decreased food availability, or allopatric geographical distribution of population co mponents and, therefore, of their associated mortality.